I'm surprised no one has mentioned Microsoft Security Essentials. I know, I know, it's Microsoft blablablah and all but honestly, *knock wood* my system has been clean since I've been using it a year and a half ago. And yes, I was using Avast and Avira prior to my SE discovery.
I have also had good luck with Microsoft Security Essentials over the past few years. I would recommend checking it out if you have never tried it.Microsoft security essentials is pretty good free antivirus.. I found that it's pretty good catching malware